Which One Is 6 Which One Is 9

In the dream, Little Bubu and her stuffed bear were playing a game of drawing numbers on the glass. Among the ten numbers 0123456789, Little Bubu could not distinguish between 6 and 9. There was a voice in the dream chasing Little Bubu: "Which of us is 6 and who is 9?" Miraculously, little Bubu could tell which one was 6 and which was 9 after waking up. Do you want to know why? Intention: distinguish which is 6 and which is 9